Generally, a conductor layer in a semiconductor device is formed by utilizing a polycide having a low resistivity. In order to make the polycide, a polysilicon film is formed on a silicon substrate, after which the impurity is implanted therein. Next, a tungsten silicide film having a W-Six structure is formed on the polysilicon film.
However, as semiconductor device integration is increased, the thickness of the polysilicon film must be decreased. In particular, the polysilicon film must be formed in a thickness of less than 500 .ANG. in a semiconductor device having over 64M DRAM. However, if the thickness of the polysilicon film is formed in a thickness of less than 1000 .ANG., the resistivity thereof increases rapidly. As a result, while it is possible to decrease the thickness of the polysilicon film, the resistivity thereof is undesirably increased.
On the other hand, the major factors in determining the resistivity of the polysilicon film are its impurity density, thickness and the grain size of the film. Assuming that the impurity density in the polysilicon film is constant in saturation, the resistance is determined according to the thickness and the grain size of the film. That is, the thickness and the grain size of the polysilicon film are directly and closely related to each other in the following manner. The thicker the polysilicon film is, the larger the grain size will be, and conversely, the thinner the polysilicon film, the smaller the grain size. Furthermore, if the grain size of the polysilicon film is small, the density of the grain in the bulk is relatively increased and the trapping phenomenon of the carrier is extreme, thereby increasing the resistivity of the polysilicon film.
Accordingly, there is a great need to decrease the resistivity of the conductor layer to increase speed, especially when the thickness of the polysilicon film is made less than 1000 .ANG..